1. Field of the Invention
The invention relates to a control circuit for adjusting the luminance of a light emitting diode by using a pulse signal.
2. Description of Related Art
Hitherto, it was general to use lamps as a light source for illuminating a console panel or the like of a vehicle. The luminance of the lamp is adjusted by a PWM (Pulse Width Modulation) signal (hereinafter, simply referred to as a pulse signal) which is supplied from a so-called illuminance control circuit. Specifically, the pulse signal from the illuminance control circuit is smoothed, a DC voltage included in the pulse signal is extracted, and a constant voltage circuit for driving the lamp is controlled by the DC voltage, thereby making a luminance adjustment of the lamp.
Since lamps are used as light source, there are problems that the electric current consumption is large, a large-size transistor has to be used in the constant voltage circuit for driving the lamps, so that it is difficult to miniaturize the luminance adjusting circuit as a whole. There also is a problem of a short lamp life due to a failures such as breakage of the filament. A further drawback is that it is difficult to obtain the maximum luminance of the lamp. Therefore, in recent years, to solve the drawback light emitting diodes are often used in place of the lamps of the light source for illumination.
Since the pulse signal for the luminance adjustment which is supplied from the illuminance control circuit is generated based on the luminance change characteristics of the lamp as a reference, there are many inconveniences. For instance, if the light source is replaced with the light emitting diode, the degree of illuminance control and the change in luminance of the light emitting diode do not coincide. In another case, the luminance of the light emitting diode does not decrease too slowly or decreases suddenly in response to a dimming control.